The invention is relative to the field of lighting in which lighting devices or luminaires are used comprising one or several light sources placed in a reflector to illuminate directly a subject, e. g. in luminaires used in television studios.
Such reflector light sources are well known which furthermore concentrate or focus the light, thus changing the angle lit. This can be achieved in several different ways.
The most common lighting device of this kind is provided with shifting means for moving backwards or forwards longitudinally along its optical axis the light source placed within the reflector so as to vary the projected pattern of light. However, such lighting devices present the following disadvantages:
When the light source is advanced to a maximum degree, the end of the lamp may protrude forward thus exposing it to potential breakage. Also a percentage of light emitted by the light source is consequently lost since it is no longer captured and reflected by the reflector. Furthermore, the light source may come in contact with accessories attached to the front of the reflector.
The housings and focusing systems of the lighting device must be made large in size in order to permit the above mentioned movement.
When the light source is moved backwards, it goes beyond the rear limit of the reflector, causing an increasing amount of light to be wasted since the reflector no longer receives this light and, therefore, cannot reflect it forward as desired.
When a small-size light source is used and when a wide-angle light pattern is desired, there is a "hole in the middle" since no light is reflected there.
Lighting devices have also been made with two light sources mounted eccentrically within a reflector and rotated around the optical axis of the reflector. The use of this type of fixture is very limited, due to the light pattern variation always being projected co-axially in the same fashion, in an uneven manner around the optical axis.
A variation of this type of lighting device has been made in which the rotational movement of two light sources is coupled to a longitudinal movement of them backwards and forwards along the optical axis of the reflector, so that the light can be more or less focused. However, its utility is extremely limited.
Lighting devices having multi-filament lamps have also been used. The filaments are in this case placed physically at different places within the lamp so that the surrounding reflector receives the light coming from different angles, thus projecting the light forward in a different manner. This system has certain disadvantages and limitations and except for automobile head lights, this system is rarely used since:
Special multi-filament lamps must be used.
The light pattern is limited and fixed by the above number of filaments within the lamp and their placement.
It is not continuously variable.
Another type of lighting device uses a reflector that has two separate sections with the same optical axis. A lamp is used in each level of the reflector and each section has different optical characteristics and diameter. So by switching on one lamp or the other, or both, the lighting effect changes. However, this device is rather complicated to produce and rather limited in use.